drm/radeon: use callbacks for ring pointer handling (v3)

Add callbacks to the radeon_asic struct to handle
rptr/wptr fetchs and wptr updates.
We currently use one version for all rings, but this
allows us to override with a ring specific versions.

Needed for compute rings on CIK.

v2: udpate as per Christian's comments
v3: fix some rebase cruft

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>